The invention discloses a construction support and method for a high-altitude skew girder steep-slope side span cast-in-place section. The support comprises three vertical support bodies and an upperload-bearing rack and is characterized in that the three vertical supports are parallel with an existing road spanned by the side span cast-in-place section, the upper load-bearing rack is supported on the three vertical supports, the front support of the three vertical supports is located between the existing road and a steep slope, and the middle support and the rear support of the three vertical supports are located on the steep slope; the upper load-bearing rack comprises a main beam and a steel-tube support. The method includes the steps of firstly, setting up the construction support anda protective scaffolding; secondly, erecting formwork; thirdly, casting the side span cast-in-place section. The construction support has the advantages that the support is reasonable in design, simple in construction and good in use effect, the three vertical supports form a stable lower support system, support stability is guaranteed, the upper load-bearing rack is formed by combining the Bailey beam and the steel-tube support, and stable support of the side span cast-in-place section is achieved while influence on the construction of other side span beam sections is avoided.